Hydrophobic coatings are useful for many applications, for example, to prevent rain from wetting-out or collecting on a windshield. Another application of hydrophobic coatings is in the field of laboratory vessels. Laboratory vessels including chambers, microtiter plates, vials, flasks, test tubes, syringes, microcentrifuge tubes, pipette tips, selectively coated microscope slides, coverslips, films, porous substrates and assemblies comprising such devices are often used to handle, measure, react, incubate, contain, store, restrain, isolate and/or transport very precise and sometimes minute volumes of liquid, often biological samples. When samples are quantitatively analyzed, it can be of critical importance that precise and representative amounts of sample are transferred, or else inaccurate results are obtained. Due to the different affinities of some materials to adhere to the walls of a laboratory vessel, qualitative analyses such as concentrations of materials may also be adversely affected if certain materials in a sample selectively adhere to operational surfaces of the vessel walls.
Unfortunately, materials typically used in the manufacture of laboratory vessels do not sufficiently repel many biological sample fluids nor do they sufficiently resist the adherence of molecular constituents of such a sample fluid. The sample fluids often wet the surface of the vessel causing residual quantities of liquid sample to cling to an operational surface of the vessel when the sample is removed. In some cases, significant quantitative and/or qualitative errors result. It is therefore desirable to provide extremely hydrophobic coatings for laboratory vessels which will reduce the wetting of the operational surfaces of the vessels and reduce clinging by even the most adherent samples so that virtually no sample remains in the vessel when poured, ejected or vacuumed therefrom.
In some laboratory techniques, it is important to restrain, isolate or limit the position of liquid samples to prescribed locations within or on a laboratory vessel, while keeping adjacent surfaces of the vessel substantially free of liquid sample. Such techniques can be used to facilitate chemical and biological reactions, as well as improving sample recovery. The prescribed locations may (1) have surfaces that are reactive, (2) have a surface that exhibits a specific affinity, (3) optimize the sample volume to area ratio, (4) restrict sample movement during at least some vessel motion, and (5) have porous surfaces.
Vessels for handling, measuring, storing and transporting liquids have previously been rendered less wettable and less adherent to fluids by application of silicone compounds to the vessel surfaces which come in contact with the fluid. For example, silane monomers and polymers have been added to polyolefins prior to injection molding, resulting in laboratory vessels with an improved repellency to many sample fluids and their constituents. These materials produce surfaces with surface energies potentially as low as 22 ergs per square centimeter. In practice, however, silane treated vessels exhibit surface energies that measure 25 to 30 dynes/cm.
Drawbacks associated with silane treatments include a continued wetting of the vessel, adherence to the vessel walls by many samples, chemical reactivity with many reagents, and a tendency for the vessel to become wettable following the common practice of autoclaving for sterilization. Silicones are known to freely migrate, leading to worries over sample integrity. Many pipette tips are plugged with porous filters to prevent sample contamination from the pipettor barrel, yet these free silicones make the pipette tips slippery and cause the filters to become loose or dislodged. Additionally, silicones must typically be added at a level of 2 percent by weight to be effective, making the cost prohibitive for many price sensitive applications.
Fluorination processes have been used to treat laboratory vessels and have resulted in vessels having interior surfaces with surface energies approaching 22 dynes/cm. These processes generally involve the full or partial replacement of superficial hydrogen by fluorine using chemical processes or the plasma polymerization of fluorine containing gases. U.S. Pat. No. 4,902,529 to Rebhan et al. discloses a plasma torch process using CF4 or SiF6 to fluorinate the interior of resin articles and containers in an attempt to eliminate the use of dangerous mixtures of fluorine and inert gases. This method is impractical, however, for treating the vast quantities of small vessels consumed by industrial, clinical and research establishments. Furthermore, improvements in performance over silicone processes are only marginal.
The plasma polymerization of perfluorobutene onto the exterior surface of various articles has been reported to produce exterior surfaces with up to 24 percent —CF3 groups, and a high percentage of —CF2— groups. Resultant surface energies of 22 to 24 dynes/cm are obtained due to the presence of cross-linkages and numerous monofluorinated carbons. Time-consuming, carefully controlled RF plasmas employing fluorine-containing monomers have also been used to reduce the wettability and adhesion of laboratory vessels, producing exterior surface energies of 12 to 15 dynes/cm and surface populations of up to about 25% by area CF3 groups on exterior non-operational surfaces. Interior operational surfaces, however, are still not reduced to below 22 dynes/cm. While these methods offer improvements over silicon-based treatments, the time, expense and equipment required are not appropriate for high commercial volume articles that are often for one-time use and require very low inherent cost.
Perfluoroalkyl polymers and carefully prepared monolayer films of perfluoroalkyl surfactants are widely recognized as having surface energies below 20 dynes/cm. FEP and PFA Teflons®, available from DuPont's Polymer Products Department, Wilmington, Del., have surface energies of 15 to 16 dynes/cm with —CF3 populations as high as 25 percent. Extruded and fused Teflon® vessels are currently manufactured for special applications involving exceptionally harsh reagents but are expected to have a long service life because of their high material cost when compared to the cost of glass or polypropylene vessels.
Fluoroalkyl polymers have been used to produce oleophobic, hydrophobic membrane surfaces that are not wetted by common organic solvents. Membranes coated with such polymers are disclosed in U.S. Pat. No. 4,954,256 to Degen et al. These membranes have surface energies ranging from about 6 to about 15 dynes/cm but require a manufacturing procedure which involves soaking a membrane with a solution containing polymerizable monomers, exposing the solution-wetted membrane to high doses of ionizing radiation, and then washing the ionized membrane with organic solvent to remove unreacted monomer. While no attempts are known to coat laboratory vessels by such a procedure, it is expected that difficulties would arise as well as high cost in coating such vessels because of the shear bulk of the polymerizable solution to be irradiated and problems with fully washing the coated vessel.
Methods of making disposable, one-time use laboratory vessels such as pipette tips can involve a substantial loss of costly solvent when a coating solution is used to form a hydrophobic coating. A need exists for a process of coating laboratory vessels at a cost of a few cents per thousand with an insignificant loss of solvent.
Recent patents may suggest the practice of solvent recovery in the application of certain branched fluoropolymers, such as Teflon AF, to articles of manufacture. U.S. Pat. No. 5,356,668 to Paton et al. and U.S. Pat. No. 5,006,382 to Squire are incorporated herein in their entireties by reference. The mere suggestion of such a recovery practice, however, does not provide a commercially viable method for coating many low cost, one-time-use articles, such as pipette tips and laboratory vessels.
Environmental concerns about pollution by volatile solvents, especially chlorine-containing materials such as perchloroethylene, have motivated significant improvement in dry cleaning equipment, resulting in significant reductions in solvent losses. Cleaning and coating equipment used in the semiconductor, plastics, and metal parts industries have made similar strides. Better seals and welded ducts account for some of the upgrades.
Operation of these machines according to their suggested protocols using fluorinated solvents, however, still results in large, expensive losses. For example, the Renzacci Company of Italy manufactures perchloroethylene-based cleaning machines that are widely recognized to be among the best in the industry in terms of minimal solvent loss. But loaded with 60,000 pipette tips in mono-filament mesh bags and using Renzacci's standard automated programs, these machines loose about 5 pounds of FC84 (3M Company, St Paul, Minn.) per cycle. This translates to solvent consumption costs of over one dollar per thousand tips. Higher boiling point fluorocarbon solvents have lower loss rates, but the solvent expense is about the same due to their higher cost per pound.
The Renzacci standard automated program partially fills a cleaning/coating tank of approximately one-half cubic meter with solvent at ambient temperature from a solvent reservoir. Articles in the tank are then tumbled in the solvent for several minutes, followed by drain, spin and spin-rinse cycles. With continued tumbling, a heat pump and a supplementary heat source (electric, steam, etc.) heat air blown through the tank, while passing air returns from the tank over chilled condensation coils where solvent vapor is liquefied and returned to the reservoir. Water is circulated through the heat pump system to remove excess heat. However, the temperature in the tank can still rise to over 50° C. and the reservoir temperature can rise to more than 30° C. At the end of the process cycle, heating is discontinued and the tank and reservoir are cooled to about 30° C. When the tank door is opened to remove the cleaned/coated articles, a small blower draws air out of the tank through a carbon filter in order to reduce the odor of remaining perchloroethylene solvent.
Unfortunately, at 30° C. the solvent FC84 has a vapor pressure of over one fifth atmosphere, and the half cubic meter tank volume contains about two pounds of solvent as dense vapor (about 14 times that of air), even without agitation. Opening the tank door results in the immediate loss of this material, at a current cost of about $45 (US). Since the machine will handle about 60,000 tips per run, the loss per thousand is about 50 cents. The carbon recovery filter is at the top of the tank and is of little practical economic value.
Additional losses accrue during the heat cycle at 50° C. when the machine fittings and seals are challenged by pressures approaching 1.2 atmospheres. Furthermore, it is apparent from other studies that low molecular weight fluorocarbon solvents, having boiling points between 80° and 120° C. are particularly “slippery” in passing through rubber and silicone gaskets and seals. No machines investigated had more aggressive containment systems for leak-free operation under these conditions.
A need exists for a method of coating large numbers of laboratory vessels which results in a very low loss of solvent at a surprising and significant cost savings.
Described by Dettre and Johnson in 1964 are phenomena related to rough hydrophobic surfaces. Dettre and Johnson developed a theoretical model based on experiments with glass beads coated with paraffin or TFE telomer. For even moderately hydrophobic surfaces (e.g. about 40 dynes/cm or less) with high levels of microscopic roughness, where the average height of bumps is close to or exceeds their average width, an aqueous liquid, especially one without surfactant activity, in contact with the surface only wets the top of the bumps, forming what is known as a “composite” air-liquid-solid interface. For example, water at rest on a surface of this kind may exhibit contact angles greater than 160 degrees. This unusual property has been practiced and is the basis for a variety of proprietary microscope slide, plate and membrane products using coatings sold by Cytonix Corporation, in Beltsville, Md. However, such products are based on Teflon® and the hydrophobic properties of difluoromethylene (—CF2—) groups, which at best exhibit surface energies of from about 18 to about 20 dynes/cm.
Hydrophobic coatings are also used on antennas and radomes. Microwave signals are significantly, and in some cases about equally, attenuated by atmospheric precipitation and by water films on antennas and radomes. Higher microwave frequencies have resulted in greater communication bandwidth, but the shorter wavelengths are even more susceptible to rain attenuation. Airports report losses of vital satellite links during heavy precipitation, and most home viewers of satellite TV are familiar with programming disruption during even light rain. As bandwidths expand commercial and private use of microwave links, the problem of rain attenuation will become even more critical.
Numerous companies have addressed the problem of water filming on microwave radomes and dishes by using hydrophobic coatings to shed water as small, microwave-transparent beads. The smooth silicones and fluoropolymer coatings allow formation of large beads that can form rivulets and films during moderate to heavy rain. As these coatings degrade over time due to sunlight and pollution, they become less hydrophobic and their effectiveness is diminished.
Several companies, notably Vellox (Salisbury, Mass.) and Boyd (Hudson, Mass.), have addressed the problem of rain fade using hydrophobic coatings that comprise micropowders of Teflon™ or fumed silica dispersed in a hardenable resin, such as an alkyd or diisocyanate. These composite coatings have good performance initially but begin to form water films in an hour or less of moderate to heavy rain. After months or just weeks of exposure to mid-latitude summer sunlight these types of coatings wet out even more quickly. Herein, mid-latitude summer sunlight is defined as average mid-day uv radiation during the summer months in regions of the United States from a latitude of 25° to a latitude of 40°, and moderate to heavy rain is defined as 1 to 6 inches per hour. Drying fully after being swamped, the Boyd Teflon™ dispersion coating CRC6040 recovers most of its previous performance, but the hydrophobic performance of the Vellox LC-410 fumed silicate coating is permanently lost. Both coatings must be reapplied every year or two.
It is believed that sunlight damages at least the surfaces of all solid polymers to some degree. Generally, unsaturated materials degrade faster than aliphatics, and aliphatics degrade faster than some fluoropolymers. But all hydrocarbon resins suffer at least superficial changes that render them more wettable; and this is also true for most fluoropolymers. The fluoropolymer exceptions are those that degrade to hydrophobic by-products, such as PTFE; but even the exceptions can be rendered somewhat more hydrophilic and receptive to adhesives by ionizing radiation treatment. Once surface damage has taken place, the new surface is a permanent feature of the solid. Tests of all common fluorinated and non-fluorinated plastics show increases in surface wettability after exposure to the equivalent of months in mid-latitude summer sunlight.
A need exists for a method of manufacturing a coating which exhibits, on all or part of an operational surface thereof, interfacial contact angles to aqueous samples of 120° and above, even as high as 160°, and surface energies well below 20 dynes/cm. According to some desirable applications, a need also exists for vessels having surface energies of below 10 dynes/cm. This need is especially acute but difficult to achieve for one-time-use vessels costing only a few dollars per thousand.
There is also a need for extremely hydrophobic coatings that are durable, for example, coatings for articles such as windshields, rainshields, and satellite and/or radar dishes, other signal receivers and transmitters, and radomes. A need exists for a composition which can provide an extremely hydrophobic and durable coating on a surface of an article. Problems associated with water film formation on radomes, and problems of radar desensitization in rain are described, for example, in the Honeywell Technical Newsletter entitled RADAR DENSENSITIZATION IN RAIN, WATER FILMS ON RADOMES, AND HYDROPHOBIC COATINGS, Nov. 2, 1998, re-published by Cytonix Corporation with permission from Honeywell, Inc., said newsletter being incorporated herein in its entirety by reference. A need exists for a coating composition for a signal transmitter or receiver, wherein the composition can be applied and form an extremely hydrophobic coating that does not interfere with signal transmission or reception.
A need also exists for a composition that forms a hydrophobic surface useful as a surface for articles which could benefit from hydrophobic properties, for example, radomes and antennas, vehicular surfaces, architectural surfaces, outdoor furniture, household goods, and kitchen and bath articles.